1. Technical Field of the Invention
The present invention relates to a piezo-driven parts feeder for conveying parts in alignment by generating vibration to a parts conveying member having a parts conveying track with a vibration generator including an elastic member having a piezoelectric element mounted thereto.
2. Description of the Prior Art
Piezo-driven parts feeders are well known which convey parts in alignment by generating vibration to a parts conveying member including a parts conveying track with a vibration generator having a piezoelectric element mounted thereto. For example, ones disclosed in JP-A-62-4118 and JP-A-9-110133 are known. The piezo-driven parts feeder described in JP-A-62-4118 aims to increase the amplitude of vibration applied to a parts conveying member by connecting an elastic plate (elastic member) of a vibrator (vibration generator) and a conveyer (parts conveying member) together with a connecting plate having a Young's modulus lower than that of the elastic plate (refer to FIG. 16). The piezo-driven parts feeder described in JP-A-9-110133 aims to decrease the height of the parts feeder by laterally arranging a vibrator (vibration generator) having a piezoelectric element bonded to an elastic plate (elastic member) (refer to FIG. 17).
With the piezo-driven parts feeder described in JP-A-62-4118, however, to achieve high-frequency resonance, the spring rigidity of the connecting plate must be increased depending on a desired resonance frequency. Therefore, it is necessary to increase the thickness of the connecting plate or to decrease the effective length. As a result, the effects of an increase in amplitude will be decreased. The spring rigidity of the elastic plate of the vibrator is also increased together with the connecting plate, increasing a stress on the elastic plate having the piezoelectric element, which is not preferable. Also, since the connecting plate and the elastic plate are arranged in series, the parts feeder is also increased in height. Furthermore, when the connecting plate is replaced for the purpose of changing resonance frequency, the elastic plate must also be replaced, not only taking a complicated labor for reassembly but also involving a great difficulty in the works of mounting the short elastic plate and the connecting plate, carrying out the setting of frequency, and fine adjustment of a mounting angle (vibration angle). During the replacement, a large load is applied to a connecting plate and a vibrator that are not being replaced, having a tendency for the connecting plate and so on to cause plastic deformation. With such a structure, the static load of the conveyor acts as a bending load directly on the vibrator, thus applying a load on the piezoelectric element.
With the piezo-driven parts feeder described in JP-A-9-110133, an elastic plate having a piezoelectric element bonded thereto is arranged linearly from the center of the parts feeder outwards (radially). Accordingly, when a vibrated body for a conveyor (moving table) rotates, a tensile stress is applied to the elastic plate because the elastic-plate fixed position is displaced from the rotation center of the vibrated body for a conveyor (refer to FIG. 17). FIG. 18 schematically illustrates the force applied to the elastic plate during the vibration of the parts feeder. As shown in the drawing, when the vibrator vibrates, the end of the elastic body mounted to the vibrated body for a conveyor through a vertical connecting member is going to shift from the point A to the point B′ with the point O′ that is the mounting position of the elastic body to the fixed side as the center. At that time, the point A on the vibrated body for a conveyor above the position at which the end of the elastic plate is mounted is going to shift to the point B with the point O that is the center of the vibrated body for a conveyor as the center. Accordingly, a radial tensile stress occurs in the elastic body having the piezoelectric element bonded thereto. Also, the elastic body is bent in S-shape in practice, causing a greater stress. Consequently, with the piezo-driven parts feeder described in JP-A-9-110133, the elastic member having the piezoelectric element tends to be subjected to an excess tensile strength, so that also the piezoelectric element is subjected to a load, thus being reduced in life, and power is wasted, thus reducing efficiency.